[PATCH v3 1/6] ubsan: Add trap instrumentation option

Kees Cook
2020-01-16T01:24:27Z

The Undefined Behavior Sanitizer can operate in two modes: warning
reporting mode via lib/ubsan.c handler calls, or trap mode, which uses
__builtin_trap() as the handler. Using lib/ubsan.c means the kernel
image is about 5% larger (due to all the debugging text and reporting
structures to capture details about the warning conditions). Using the
trap mode, the image size changes are much smaller, though at the loss
of the "warning only" mode.

In order to give greater flexibility to system builders that want
minimal changes to image size and are prepared to deal with kernel code
being aborted and potentially destabilizing the system, this introduces
CONFIG_UBSAN_TRAP. The resulting image sizes comparison:

text data bss dec hex filename
19533663 6183037 18554956 44271656 2a38828 vmlinux.stock
19991849 7618513 18874448 46484810 2c54d4a vmlinux.ubsan
19712181 6284181 18366540 44362902 2a4ec96 vmlinux.ubsan-trap

CONFIG_UBSAN=y: image +4.8% (text +2.3%, data +18.9%)
CONFIG_UBSAN_TRAP=y: image +0.2% (text +0.9%, data +1.6%)

Additionally adjusts the CONFIG_UBSAN Kconfig help for clarity and
removes the mention of non-existing boot param "ubsan_handle".

Suggested-by: Elena Petrova <lenaptr@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Dmitry Vyukov <dvyukov@google.com>
---
lib/Kconfig.ubsan | 22 ++++++++++++++++++----
lib/Makefile | 2 ++
scripts/Makefile.ubsan | 9 +++++++--
3 files changed, 27 insertions(+), 6 deletions(-)

diff --git a/lib/Kconfig.ubsan b/lib/Kconfig.ubsan
index 0e04fcb3ab3d..9deb655838b0 100644
--- a/lib/Kconfig.ubsan
+++ b/lib/Kconfig.ubsan
@@ -5,11 +5,25 @@ config ARCH_HAS_UBSAN_SANITIZE_ALL
config UBSAN
bool "Undefined behaviour sanity checker"
help
- This option enables undefined behaviour sanity checker
+ This option enables the Undefined Behaviour sanity checker.
Compile-time instrumentation is used to detect various undefined
- behaviours in runtime. Various types of checks may be enabled
- via boot parameter ubsan_handle
- (see: Documentation/dev-tools/ubsan.rst).
+ behaviours at runtime. For more details, see:
+ Documentation/dev-tools/ubsan.rst
+
+config UBSAN_TRAP
+ bool "On Sanitizer warnings, abort the running kernel code"
+ depends on UBSAN
+ depends on $(cc-option, -fsanitize-undefined-trap-on-error)
+ help
+ Building kernels with Sanitizer features enabled tends to grow
+ the kernel size by around 5%, due to adding all the debugging
+ text on failure paths. To avoid this, Sanitizer instrumentation
+ can just issue a trap. This reduces the kernel size overhead but
+ turns all warnings (including potentially harmless conditions)
+ into full exceptions that abort the running kernel code
+ (regardless of context, locks held, etc), which may destabilize
+ the system. For some system builders this is an acceptable
+ trade-off.

config UBSAN_SANITIZE_ALL
bool "Enable instrumentation for the entire kernel"
diff --git a/lib/Makefile b/lib/Makefile
index 93217d44237f..3114ef1727f8 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -275,7 +275,9 @@ quiet_cmd_build_OID_registry = GEN $@
clean-files += oid_registry_data.c

obj-$(CONFIG_UCS2_STRING) += ucs2_string.o
+ifneq ($(CONFIG_UBSAN_TRAP),y)
obj-$(CONFIG_UBSAN) += ubsan.o
+endif

UBSAN_SANITIZE_ubsan.o := n
KASAN_SANITIZE_ubsan.o := n
diff --git a/scripts/Makefile.ubsan b/scripts/Makefile.ubsan
index 019771b845c5..668a91510bfe 100644
--- a/scripts/Makefile.ubsan
+++ b/scripts/Makefile.ubsan
@@ -1,5 +1,10 @@
# SPDX-License-Identifier: GPL-2.0
ifdef CONFIG_UBSAN
+
+ifdef CONFIG_UBSAN_ALIGNMENT
+ CFLAGS_UBSAN += $(call cc-option, -fsanitize=alignment)
+endif
+
CFLAGS_UBSAN += $(call cc-option, -fsanitize=shift)
CFLAGS_UBSAN += $(call cc-option, -fsanitize=integer-divide-by-zero)
CFLAGS_UBSAN += $(call cc-option, -fsanitize=unreachable)
@@ -9,8 +14,8 @@ ifdef CONFIG_UBSAN
CFLAGS_UBSAN += $(call cc-option, -fsanitize=bool)
CFLAGS_UBSAN += $(call cc-option, -fsanitize=enum)

-ifdef CONFIG_UBSAN_ALIGNMENT
- CFLAGS_UBSAN += $(call cc-option, -fsanitize=alignment)
+ifdef CONFIG_UBSAN_TRAP
+ CFLAGS_UBSAN += $(call cc-option, -fsanitize-undefined-trap-on-error)
endif

# -fsanitize=* options makes GCC less smart than usual and
--
2.20.1